The present invention relates generally to a dry pipe sprinkler system and, more particularly, to such a system which operates using low pressure gas charged fluid flow lines and which includes circuitry for rapidly detecting an open sprinkler head condition and for discriminating such a condition from a pressurized gas leak in the charged dry pipe.
Dry pipe sprinkler systems are well-known in the prior art. A dry pipe sprinkler system includes a dry pipe sprinkler grid containing a plurality of normally closed sprinkler heads. The dry pipe sprinkler grid is connected via fluid flow lines to a dry pipe valve or primary water supply valve which has a dry output side facing the fluid flow lines and a wet input side facing a pressurized source of service water. In operation, the fluid flow lines and sprinkler grid fluid flow lines are filled or charged with a pressurized gas, such as air. Current industrial dry pipe systems generally charge the fluid flow lines to about 35 to 40 psig. The sprinkler heads typically include normally closed temperature-responsive valves. If heated sufficiently, the normally closed valve of the sprinkler head opens, thereby allowing the pressurized air to escape from the fluid flow lines. When the air pressure in the fluid flow lines drops below a predetermined value, a mechanism causes the primary water supply valve to open, thereby allowing the service water to flow into the fluid flow lines of the dry pipe sprinkler grid (displacing the air therein), and through the open sprinkler head to extinguish the fire or smoke source, or to minimize any damage therefrom. Water flows through the system and out the open sprinkler head (and any other sprinkler heads that subsequently open), until the sprinkler head closes itself, if automatically resetting, or until the water supply is turned off.
A dry pipe sprinkler system is distinguished from a wet pipe sprinkler system. In a wet pipe sprinkler system, the fluid flow lines are prefilled with water, and water is retained in the sprinkler grid by the valves in the sprinkler heads. As soon as a sprinkler head opens, the water in the sprinkler grid immediately flows out of the sprinkler head. In a wet pipe sprinkler system, the primary water valve is the main shut-off valve, which is in the normally open state.
There are a large number of different mechanisms and techniques for causing a dry pipe sprinkler system to go "wet" (i.e., to cause the primary water supply valve to open and allow the water to fill the fluid flow supply lines). In one known technique, after a sprinkler head opens, the pressure difference between the air pressure in the fluid flow lines and the water supply pressure on the wet side of the primary water supply valve must reach a specific hydraulic unbalance before the primary water supply can open. It may take up to 30 seconds to reach this state, depending upon the volume of the entire system. If the system is large and/or if the system is charged to a typical pressure such as 40 psig, a considerable volume of air must escape or be expelled from the open sprinkler head before the specific hydraulic unbalance is reached to open the primary water valve and then to force water through the grid through the remaining pressurized gas, to finally discharge through the open sprinkler.
Dry pipe sprinkler systems suffer from other problems. They are susceptible to false alarms from ambient temperature-induced expansion and contraction of the pressurized air in the fluid flow lines and from gas leaks in the fluid flow lines. For example, the pressurized gas may contract to a degree that triggers opening of the primary water valve. The system must then be drained and recharged.
There is significant interest in installing sprinkler systems in residential units for protection of residents and property. Wet pipe systems are not well-suited to residential use. Builders desire to hide the fluid flow lines and may pass them through unheated or poorly heated areas where they are susceptible to freezing and bursting in cold weather. Furthermore, there is a reluctance to install dry pipe sprinkler systems in residential environments because of the deficiencies discussed above, namely the relatively slow response time of the systems, and the problems associated with false alarms. Also, existing dry pipe systems are expensive to install and maintain due, in part, to the necessity to maintain a high pressure gas charged system.
Accordingly, there is a need for a dry pipe sprinkler system which has a quick response time, which is capable of accurately discriminating between false alarm conditions, an open sprinkler condition and a pressurized gas leak condition, and which can be operated at a reduced fluid flow line pressure level. The present invention fulfills such needs.